I've replaced both the pickups and the preamp in my Century J.
I did the pickups first, and the difference was very substantial. I went with the Nordstrand NJ5F single coils, and they REALLY opened the bass up. I could have just left it alone and been completely happy with it........
But, I decided to change out the preamp as well, and swapped in a Nordstrand 3b-4b preamp. The difference there was more subtle than the pickup swap, but it was a slight upgrade (to my ears). The boost on the Nordstrand preamp seems to be more "musical", for lack of a better term. It really smoothed out the top end in the bass----still nice and bright, but a bit less harsh. Low boost doesn't seem to go from flat to holy poopie as quickly as the Bart pre did. Mids are very usable, but I usually don't have to boost mids on that bass. The passive tone control is something I probably won't really use, but, it's on there if I want it I guess.